1. Field of the Invention
This invention relates generally to semiconductor devices and more particularly to a package housing for power transistors or power integrated circuits.
2. Discussion of the Related Art
A typical package housing for power semiconductor devices is illustrated by the TO-220 package. This package housing comprises a metal plate partially embedded in a synthetic resin or plastic body exposing a major surface of the plate. One or more semiconductor chips forming the operative part of the device, are secured to the other major surface of the plate and are in good thermal contact therewith to ensure an external sink or dissipating element for the heat produced by the chip in operation. Rigid metal conductors serving as connecting terminals for the device are connected to metallized zones of the semiconductor chips by thin metal wires and are also partly embedded in the resin body. The unembedded part of the plate is formed with a through-hole enabling the plate to be secured to a heat dissipating element by a screw or rivet.
The known package housing of the kind described, although having many advantages over others of different types such as low production costs, do not ensure adequate mechanical strength in all applications. This disadvantage is principally the result of the fact that the metal and the resin (or plastic) used for the package housing have different physical characteristics from one another. The different physical characteristics prevent a completely rigid securing of the resin body to the metal plate. During some phases of the process for producing the package housing and, more particularly, when the device is secured to a heat dissipating element by the user, there is a risk of the semiconductor chip being subjected to mechanical stresses likely to damage it. This risk is very high when the package housing is screwed to a dissipating element which is not completely flat, or which is slightly concave.
Various solutions for the problem have been proposed. For instance, an attempt has been made to use an intermediate layer of a material which adheres well both to the resin and to the metal and which is highly resilient. The different expansions of the metal plate and of the resin body are thus compensated for and the mechanical stresses transmitted to the semiconductor chip are reduced. It has also been proposed to use claws or grooves or through-holes in the metal plate to improve the securing of the resin body to the metal plate. However, solutions of this kind do not always solve the problem. In addition, these solutions complicate the devices and manufacturing processes and therefore increase the cost of the resulting electronic component.